In certain environments, it may be difficult to connect an electrical circuit to power supply cables, for example in hostile environments or in mechanisms in motion. To overcome this problem, micromechanical devices for converting vibration energy into electrical energy are known. These devices form microsystems generally attach to vibrating supports such as machines or vehicles. One known technique uses a resonant system to amplify a mechanical vibration of a support and convert the amplified motion into electricity. The electrical circuit can thus be powered without using cables coming from the exterior.
One of the known principles for converting mechanical vibration energy into electrical energy is based on the vibrational excitation of a beam provided with piezoelectric elements. Such a beam generally has a core with a first end embedded in a vibrating support. A mobile mass is fixed to the second end of the core. A piezoelectric element is fixed to the upper face of the core and another piezoelectric element is fixed to the lower face of the core. An electrical circuit is connected to the terminals of the piezoelectric elements which are placed electrically in series or in parallel. The piezoelectric elements are intended for converting the mechanical energy transmitted by the mobile mass into electrical energy.
Another known principle used to convert mechanical vibration energy into electrical energy is based on an electrostatic system. The electrostatic system uses a variable capacitance to convert the mechanical vibration energy into electrical energy.
The recurrent problems that arise with the different types of energy-harvesting microstructures are the small quantity of energy harvested, the low efficiency of energy harvesting when the frequency of the vibration moves away from the inherent mechanical resonance frequency of the system and the narrowness of the bandwidth of vibration energy harvesting.
With a piezoelectric structure, in order to increase the bandwidth of the frequencies of vibrations that generate electrical energy, the mechanical resonance frequency of the resonant system can be modified by controlling the polarization of a first piezoelectric element, as presented in the thesis by David Charnegie, “Frequency tuning concepts for piezoelelectric cantilever beams and plates for energy harvesting”. The electrical energy is then harvested at a second piezoelectric element. The stiffness of the first piezoelectric element can thus be modified actively to modify the stiffness of the beam and thus influence the frequency of the mechanical resonance. To this end, a variable capacitance is connected in parallel with a first piezoelectric element. Since the second piezoelectric element is not connected to the variable capacitance, this variable capacitance does not affect the harvesting of electrical energy in the electrical load.
Thus, it is possible to set up an automatic control over the mechanical resonance frequency of the system. An automatic control of this kind proves to be necessary when a system from which vibration energy is extracted has a variable vibration frequency. An example of such a system is a motor vehicle in which the rotation speed of the engine or the rotation speed of the wheels undergoes great variations.
While such a system enables the mechanical resonance frequency to be adapted to the frequency of the vibration source, it does not yet sufficiently optimize energy harvesting. In particular, this system especially does not make it possible to ensure that the electrical damping of the electrical load is equal to the mechanical damping of the beam. In addition, it is difficult to envisage the modification of the impedance of the load in order to modify the mechanical resonance frequency without disturbing the operation of this load. Nor does this system make it possible to optimize the energy harvested from the piezoelectric elements.
The document “Tunable Capacitor Based on Polymer-Dispersed Liquid Crystal for Power Harvesting Microsystems” published on Jan. 10, 2008, describes an adjustable capacitor based on PDLC liquid crystal. The document describes the integration of such a capacitor in a device for harvesting vibration energy. The efficiency of the energy harvesting device is improved by adjusting the resonance frequency of a piezoelectric beam to the ambient vibration frequency, using the adjustable capacitor.